1. Field of the Invention
The present invention relates to a light amplifier device that performs an addition or subtraction operation on currents output from a plurality of light-receiving elements and that outputs a voltage amplified according to the current resulting from the addition or subtraction operation. More particularly, the present invention relates to a light amplifier device for use in an optical pickup device.
2. Description of the Prior Art
In an optical pickup device for optical discs, a current signal output from a light-receiving section is used not only for data reading but also for servo control to achieve focusing (focusing of a reading light beam) and tracking (positioning of the reading light beam), both essential for correct reading of data. To achieve this, the light-receiving section is usually provided with not a single light-receiving element but a plurality of light-receiving elements arranged next to one another so that the servo control is achieved on the basis of the differences in the amount of light received by the individual light-receiving elements when a spot of light is incident on the light-receiving section.
On the other hand, for the purpose of data reading, to minimize read errors, a signal obtained by adding together all the current signals output from the individual light-receiving elements is used. Formerly, this addition operation has been performed by a signal processing integrated circuit provided outside an optical pickup device. Recently, however, such an addition operation has come to be performed increasingly by a light amplifier provided within an optical pickup device. One reason is that read and write rates have recently been increasing dramatically. Another reason is that, for reproduction from a plurality of types of optical discs, a laser beam having a plurality of frequencies has come to be used, which lowers the S/N ratio of the output signals from the light-receiving elements and accordingly makes less negligible the noise induced in the leads connecting the light-receiving elements to the signal processing integrated circuit that processes the output signals of the light-receiving elements. Still another reason is that further reduction of costs and electric power consumption has been expected in optical pickup devices.
FIG. 6 shows the configuration of a conventional light amplifier that adds together the current signals output from a plurality of light-receiving elements. The cathode of a photodiode D1 is connected to the input terminal of a transimpedance amplifier 26, and the cathode of a photodiode D2 is connected to the input terminal of a transimpedance amplifier 27. The anodes of the photodiodes D1 and D2 are kept at the ground potential. It is to be noted that a transimpedance amplifier denotes an amplifier that converts a current signal it receives into a voltage signal it outputs.
The output terminal of the transimpedance amplifier 26 is connected to one end of a resistor R6, and the output terminal of the transimpedance amplifier 27 is connected to one end of a resistor R7. The other ends of the resistors R6 and R7 are connected together, and the node n2 between them is connected to the input side of a non-inverting amplifier 28. The output side of the non-inverting amplifier 28 is connected to a terminal 4.
The non-inverting amplifier 28 is composed of an operational amplifier OP2 and resistors R8 and R9. The non-inverting input terminal of the operational amplifier OP2 serves as the input side of the non-inverting amplifier 28. One end of the resistor R8 and one end of the resistor R9 are connected to the inverting input terminal of the operational amplifier OP2, and the other end of the resistor R9 is kept at the ground potential. The other end of the resistor R8 is connected to the output terminal of the operational amplifier OP2, and the node between them serves as the output side of the non-inverting amplifier 28.
The output voltage VOxe2x80x2 of the light amplifier configured as described above is given as follows. Let the output voltage of the transimpedance amplifier 26 be V26, the output voltage of the transimpedance amplifier 27 be V27, and the potential at the node n2 be Vn2. Then, the current I fed to the non-inverting input terminal of the operational amplifier OP2 is given by equation (1) below, where r6 represents the resistance of the resistor R6 and r7 represents the resistance of the resistor R7.
I=(V26xe2x88x92Vn2)/r6+(V27xe2x88x92Vn2)/r7xe2x80x83xe2x80x83(1)
The relationship between the voltage Vn2 and the output voltage VOxe2x80x2 is expressed by equation (2) below, where r8 represents the resistance of the resistor R8 and r9 represents resistance of the resistor R9.
VOxe2x80x2=(1+r8/r9)xc3x97Vn2xe2x80x83xe2x80x83(2)
When equations (1) and (2) are integrated together, the output voltage VOxe2x80x2 is given by equation (3) below. Here, the term including the current I, which is a very small current, is approximated as zero.
xe2x80x83VOxe2x80x2=(1+r8/r9)xc3x97(r7xc3x97V26+r6xc3x97V27)/(r6+r7)xe2x80x83xe2x80x83(3)
When the resistance r9 of the resistor R9 is set as defined by equation (4) below, and equations (3) and (4) are integrated together, then the output voltage VOxe2x80x2 is given by equation (5) below.
r9=(r6xc3x97r7)/(r6+r7)xe2x80x83xe2x80x83(4)
VOxe2x80x2=(1+r8/r9)xc3x97r9xc3x97(V26/r6+V27/r7)xe2x80x83xe2x80x83(5)
In equation (5), V26/r6 can be regarded as the output current of the transimpedance amplifier 26, and V27/r7 can be regarded as the output current of the transimpedance amplifier 27. Moreover, the voltage V26 is the result of the conversion of the output current of the photodiode D1 by the transimpedance amplifier 26, and the voltage V27 is the result of the conversion of the output current of the photodiode D2 by the transimpedance amplifier 27. Hence, equation (5) shows that the output voltage VOxe2x80x2 is a voltage amplified according to the value obtained by adding together the currents output from the photodiodes D1 and D2.
In the conventional light amplifier shown in FIG. 6, if the gain of the operational amplifier OP2 is assumed to be A0, the loop gain Txe2x80x2 of the non-inverting amplifier 28, which is a negative feedback amplifier, is given by equation (6) below.
Txe2x80x2=A0xc3x97r9/(r9+r8)xe2x80x83xe2x80x83(6)
Here, an attempt to increase the gain of the conventional light amplifier shown in FIG. 6 by increasing the amplification factor of the current signals fed to the transimpedance amplifiers 26 and 27 results, since the resistance r9 is set as defined by equation (4), in reducing the resistance r9, with the result that, as equation (6) clearly shows, the loop gain Txe2x80x2 of the non-inverting amplifier 28 is reduced.
In the conventional light amplifier shown in FIG. 6, its characteristics are enhanced by a factor of [(loop gain)/(gain after negative feedback)] by configuring the non-inverting amplifier 28 as a negative feedback amplifier, as compared with a case where no negative feedback is present. However, as described above, when the amplification factor of the current signals fed to the transimpedance amplifiers 26 and 27 is increased with a view to increasing the gain of the conventional light amplifier shown in FIG. 6, the loop gain Txe2x80x2 of the non-inverting amplifier 28 is reduced, and thus the characteristics of the non-inverting amplifier 28 are degraded. This makes it impossible to achieve a high gain and a wide band width with the conventional light amplifier shown in FIG. 6.
Incidentally, Japanese Patent Application Laid-Open No. H2-301879 discloses an adder that outputs a voltage amplified according to the current obtained by adding together the output currents of a plurality of amplifiers (conductance amplifiers or variable conductance amplifiers) provided within the adder itself. However, the amplifiers provided within this adder receive voltages as their inputs, and therefore this adder cannot be used as a light amplifier to which the output currents from light-receiving elements are fed.
An object of the present invention is to provide a high-gain, wide-band-width light amplifier that performs an addition or subtraction operation on currents output from a plurality of light-receiving elements and that outputs a voltage amplified according to the current resulting from the addition or subtraction, and to provide an optical pickup device employing such a light amplifier.
To achieve the above object, according to the present invention, a light amplifier is provided with: a plurality of light amplifier circuits, each having a light-receiving element that outputs a current according to the intensity of light received and a current amplifier that outputs a current by amplifying the output current of the light-receiving element; and a first transimpedance amplifier whose input terminal is connected to the node at which the output terminals of the individual current amplifiers are connected together.